Electrolytic method of making concentrated hydroxide solutions by sequential use of 3-compartment and 2-compartment electrolytic cells having separating compartment walls of particular cation-active permselective membranes

ABSTRACT

A concentrated hydroxide solution containing over 250 g./l. of sodium hydroxide is made by a two-cell aqueous sodium chloride electrolysis process wherein, in a first cell, containing at least three (and preferably three) compartments, with a buffer compartment separated from adjoining anode and cathode compartments by walls of a cation-active permselective membrane, which is of a hydrolyzed copolymer of a perfluorinated hydrocarbon and fluorosulfonated perfluorovinyl ether or of a sulfostyrenated perfluorinated ethylene propylene polymer, a concentrated sodium hydroxide solution is made in the cathode compartment, a dilute sodium hydroxide solution is produced in a buffer compartment, to which compartment water is added during the electrolysis and chlorine is generated in the anode compartment, after which the dilute hydroxide is fed to the cathode compartment of a two-compartment electrolytic cell in which sodium chloride solution is electrolyzed in the anode compartment and concentrated sodium hydroxide solution, at a concentration of more than 250 g./l., is withdrawn from the cathode compartment. The caustic efficiency of the combined process is above 70%, which is normally unattainable using only a two-compartment membrane cell and yet, the sodium hydroxide solution resulting is at a high concentration.

United States Patent Cook, Jr. et a1.

[451 Aug. 12, 1975 Inventors: Edward H. Cook, Jr., Lewiston;

Alvin T. Emery, Youngstown, both of N.Y.

Hooker Chemicals & Plastics Corporation, Niagara Falls, N.Y.

Filed: Nov. 1, 1973 Appl. No.: 411,619

[73] Assignee:

[52] US. Cl. 204/98 [51] Int. Cl. C0ld 1/06; COlb 7/06 [58] Field of Search 204/98, 128, 296

[56] References Cited UNITED STATES PATENTS 2,967,807 l/l96l Osborne et a1. 204/128 3,222,267 12/1965 Tirrell et a1 204/98 3,282,875 11/1966 Connolly et a]. 260/296 3,341,366 9/1967 l-lodgdon et al. 136/86 3,496,077 2/1970 Cooper 204/98 3,718,551 2/1973 Martinsons 204/98 3,773,634 3/1972 Stacey et a1. 204/98 FORElGN PATENTS OR APPLlCATlONS 1,184,321 3/1970 United Kingdom OTHER PUBLICATIONS New Product Information from Research & Development Div. Plastics Dept, E. l. Dupont De Nemours & Co., XR Perfluorosulfonic Acid Membranes, 10- l-69, PP- l-4.

Primary Examiner-R. L. Andrews Attorney, Agent, or F irrn Peter F. Casella; Donald C. Studley [57] ABSTRACT A concentrated hydroxide solution containing over 250 g./l. of sodium hydroxide is made by a two-cell aqueous sodium chloride electrolysis process wherein, in a first cell, containing at least three (and preferably three) compartments, with a buffer compartment separated from adjoining anode and cathode compartments by walls of a cation-active permselective mem brane, which is of aliydrolyzed copolymer of a perfluorinated hydrocarbon and fluorosulfonated perfluorovinyl ether or of a sulfostyrenated perfluorinated ethylene propylene polymer, a concentrated sodium hydroxide solution is made in the cathode compartment, a dilute sodium hydroxide solution is produced in a buffer compartment, to which compartment water is added during the electrolysis and chlorine is generated in the anode compartment, after which the dilute hydroxide is fed to the cathode compartment of a twocompartment electrolytic cell in which sodium chloride solution is electrolyzed in the anode compartment and concentrated sodium hydroxide solution, at a concentration of more than 250 g./l., is withdrawn from the cathode compartment. The caustic efficiency of the combined process is above 70%, which is normally unattainable using only a two-compartment membrane cell and yet, the sodium hydroxide solution resulting is at a high concentration.

9 Claims, 1 'Drawing Figure LOW CONC.

NuOH SOL'N.

HIGH CONCENTRATION NuOH SOLUTION uvonoeeu men coNc. 5 15-225 NaoH SOL'N.

1 ELECTROLYTIC METHOD OF MAKING CONCENTRATED HYDROXIDE SOLUTIONS BY SEQUENTIAL USE OF -COMPARTMENT AND Z-COMPARTMENT ELECTROLYTIC CELLS HAVING SEPARATING -COMPARTMENT WALLS OF PARTICULAR CATION-ACTIVE PERMSELECTIVE MEMBRANES This invention relates to the electrolytic manufacture of hydroxide solutions. More specifically, it is of a process for making alkali metal hydroxides in concentrated liquid solution form by the electrolysis of aqueous alkali metal halide solutions in two different types of electrolytic cells, each of which'utilizes one or more cation-active permselective membranes of a particular type.

Chlorine and caustic are essential and large volume commodities which are required basic chemicals in all industrial societies. They are commercially produced by electrolysis of aqueous salt solutions and a major proportion of such production is by diaphragm cells. Such cells have been improved by incorporation therein of dimensionally stable anodes, which include noble metals, alloys or oxides thereof or mixtures thereof, on valve metals.

The concept of employing permselective diaphragms to separate anolyte from catholyte during electrolysis is not a new one and plural compartment electrolytic cells have been suggested which employ one or more of such membranes. Recently, improved membranes have been described which are of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. In some experiments these membranes have been employed between the anolyte and buffer zones of chlorinecaustic cells of have been utilized to separate anolyte and catholyte zones of such cells. Yet, although the electrolysis of aqueous salt solutions is a technologically advanced field of very great commercial interest in which much research is performed and although the importance of improving manufacturing methods therein is well recognized, before the present invention there had not been described such an improved process by which high strength, lowchloride content caustic solutions could be made at reasonably high caustic current efficiencies.

In another patent application of the present inventors, filed concurrently herewith and entitled Electrolytic Method for the Simultaneous Manufacture of Concentrated and Dilute Aqueous Hydroxide Solutions there is described the use, for the production of sodium hydroxide, of electrolytic cells having at least three compartments, including a buffer compartment, with the present cation-active permselective membranes separating the buffer and other compartments. However, by such a method there is produced in the buffer compartment a dilute sodium hydroxide and although this material is valuable, because of the presence of a large proportion of water therein shipping costs are often prohibitive. The present method allows the utilization of the dilute caustic inthe manufacture of a more concentrated sodium hydroxide solution so that, even if there is no nearby plant or other installation which can utilize the dilute caustic enough water has been removed from it to allow it to be marketed as an article of commerce and to be economically shipped over long distances.

In accordance with the present invention a method for electrolytically manufacturing concentrated hydroxide solutions containing over 250 g./l. of sodium hydroxide or equivalent hydroxide comprises making concentrated and dilute aqueous hydroxide solutions simultaneously by electrolyzing an aqueous solution containing halide ions in an electrolytic cell having at least three compartments therein, an anode, a cathode, at least two cation-active permselective membranes, of a polymeric materialselected from the group consisting of a hydrolyzed copolymer of a'perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated" ethylene proplyene polymer, defining anode and cathode side walls of a buffer compartment or compartments between anode and cathode compartments, and such walls, with walls thereab'out, defining anode and cathode compartments, while adding waterwto the buffer compartment at such a rate as to produce a dilute hydroxide solution therein at the same time that a more concentrated hydroxide solution, containing over 250 g./l. of sodium hydroxide or equivalent is produced in the cathode compartment, While maintaining a high caustic efficiency, removing the dilute hydroxide from the buffer compartment and feeding it to the cathode compartment of a two-compartment electrolytic cell, having anode and cathode compartments separated by a cation-active permselective membrane of a polymeric material selected from-the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonatedperfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, in which cell, in the anode compartment thereof, an aqueous solution containing halide ions is electrolyzed, and withdrawing from the catholyte compartment of the two-compartment cell concentrated hydroxide solution containing more than 250 g./l. of-sodium hydroxide or equivalent, so that the cathode compartment efficiency of. the combined processes is above 70 In preferred embodiments of the invention the permselective membranes are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FSO CF C- F OCF(CF )CF OCF=CF hereafter called PSEPVE, which polymer has an equivalent weight of about 900 to 1,600, only two such membranes are employed and the membranes are mounted on networks of supporting material such as polytetrafluoroethylene, perfluori nated ethylene propylene polymer, polypropylene, asbestos, titanium, tantalum, niobium or noble metals.

The inventionwill be more readily understood by reference to the following descriptions of embodiments thereof, taken in conjunction with. the drawing of means for effecting the invented processes.

I IN THE DRAWING droxide solutions is illustrated, other halide-forming cations may also be employed and in some instances bromine may be at least partially substituted for chlorine in the halide.

In the FIGURE electrolytic cell ll includes outer wall 13, anode 15, cathode 17 and conductive means 19 and 21 for connecting the anode and the cathode to sources of positive and negative electrical potentials, respectively. Inside the walled cell permselective membranes 23 and 25 divide the volume into anode or anolyte compartment 27, cathode or catholyte compartment 29 and buffer compartment 31. An aqueous solution of alkali metal halide, preferably acidic, is fed to the anolyte compartment through line 33, from saturator 35. During electrolysis chlorine gas is removed from above the anode compartment through line 37 and hydrogen gas is correspondingly removed from above the cathode compartment through line 39. More concentrated hydroxide solution is withdrawn from cathode compartment 29 through line 41 while the corresponding solution of lower concentration is withdrawn from the buffer compartment through line 43 and is delivered to the cathode compartment 45 of twocompartment cell 47. Water may be added to buffer compartment 31 of three-compartment cell 11 through line 49 to maintain the desired concentration of caustic in that compartment so as to maintain a high current efficiency, cathode efficiency, cathode compartment efficency, caustic efficiency or sodium ion efficiency (all such terms being interchangeable) by limiting the transmission of hydroxyl ions to the anolyte through membrane 23. Solid sodium chloride or other source of chloride ions may be fed to saturator through line 51 to raise the chloride concentration in the feed to the cells. The anolytes may be recirculated back to the saturator for addition of salt to maintain the desired concentration thereof in the anolyte. Proportioning valve 53 controls the flow of rejuvenated electrolyte to anolyte compartments 27 and 55 through lines 33' and 75.

In cell 47 anode 57 is connected to a source of positive electrical potential via conductor 59 and cathode 61 is similarly connected via a corresponding conductor 63. Cationactive permselective membrane 65 separates compartments and 55. Low concentration sodium hydroxide solution from buffer compartment 31 passes through line 67 into cathode compartment 45 to add its hydroxyl ion content to that produced in the two-compartment cell by electrolysis thereof. Thus, a high concentration sodium hydroxide solution is produced in cathode compartment 45 and is withdrawn from it through line 69. Chlorine and hydrogen are withdrawn from the anode and cathode compartments, respectively, through lines 71 and 73.

The high concentration sodium hydroxide solution produced may be sold, employed in chemical reactions, e.g., making chlorate, or may be evaporated to still higher concentrations. The chlorine may be reacted with the caustic to form hypochlorite or chlorate or may be reacted with hydrogen made to produce hydrochloric acid. Otherwise, it is salable, usually after liquefaction, in which process any oxygen present is removed. The chlorine, hydrogen and high concentration caustic solution streams from the three-compartment and two-compartment cells may be kept separate or may be combined. In preferred operations anolyte will be recirculated back to the saturator and if the chlorate concentration therein becomes too high, due to reaction of chlorine with caustic in the anode compartment of the twocompartment cell, some of the anolyte may be sent to a separator or crystallizer for removal of the chlorate, preferably, in crystalline form, or it may be fed as a chlorate cell electrolyte.

In the process described chloride-free, high strength caustic solution can be made at a high caustic current efficiency, e.g., over or in the threecompartment cell, due to the interposition of the buffer compartment, which diminishes migration of hydroxyl ions to the anolyte. Thus, the chlorine produced contains less oxygen and the electric power is utilized more for the production of caustic than in two-compartment cells. In the two-compartment cell caustic current efficiencies of only 50 or 60% (or more) are obtainable but there is no dilute caustic byproduct, all of the caustic solution being produced as saltfree high concentration caustic. The overall efficiency of the process is generally over 70% and often over 75%, a startling increase over the 50 or 60% efficiencies obtainable when utilizing the two-compartment cell alone to make strong caustic.

In the drawing the feed from one three-compartment cell is shown going to a single two-compartment cell. For simplicity of presentation the drawing is shown thus but in practice, to balance the various streams and obtain best properties in the product and highest efficiencies, a plurality of three-compartment cells may be headed together, both with respect to feeds and products, and the products may be fed to a plurality of twocompartment cells. In practice it is found that it is preferred to utilize from one to two three-compartment cells per two-compartment cell, with the ratio preferably being from 1.3 to 1.7 and most preferably about The high concentration catholyte hydroxide removed is at a concentration over 250 g./l., preferably over 350 g./l. with a maximum of about 450 g./l. Most preferably, it is of 375 to 425 g./l., e.g., 400 g./l. The dilute caustic taken off from the buffer compartment is at a concentration over 20 g./l. and is usually in the 50 to 200 g./l. range, preferably being over 75 g./l., more preferably being from lOO to g./l. and most preferably being about l25 g./l.

The selective ion-passing effects of cationic membranes have been noted in the past but the membranes of this invention have not been employed in the present prcesses before and their unexpectedly beneficial effects have not been previously obtained or suggested. Thus, with the use of a comparatively thin membrane, preferably supported as described herein, several years of operation under commercial conditions are obtainable without the need for removal and replacement of the membrane, while it efficiently prevents undesirable migration of chloride ions from the anolyte to the catholyte. Simultaneously, it prevents hydrogen formed on the cathode side from escaping into the halogen formed on the anode side and prevents hydrogen from infiltrating the chlorine and producing an explosive mixture. In this respect the present membranes are superior to prior art membranes because they are more impervious to the passage of hydrogen, even in comparatively thin films, than are various other known polymeric materials.

Although the preferred embodiments of the invention utilize a pair of the described membranes to form the three compartments of the present three compartment cells it will be evident that a greater num ber of compartments, e.g., 4 to 6, including plural buffer zones, may be employed. Similarly, also, while the cell compartments of both types of cells will usually be separated by flat membranes and will usually be of substantially rectilinear or parallelepipedal construction, various other shapes, including curves, e.g., ellipsoids, and irregular surfaces, e.g., sawtoothed or plurally pointed walls, may also be utilized. In another variation of the invention the buffer zone(s), formed by the plurality of membranes, will be between bipolar electrodes, rather than the monopolar electrodes which are described herein. Bipolar electrodes may also be employed for the twocompartment cells. Those of skill in the art will know the variations in structure that will be made to accommodate bipolar, rather than monopolar electrodes, and therefore, these will not be described in detail. Of course, as is known in the art, pluralities of the individual cells will be employed in multi-cell units, often having common feed and productmanifolds and being housed in unitary structures. Again, such constructions are known to those in the art and need not be discussed herein.

For most satisfactory and efficient operations the volume of the buffer compartment(s) will usually be from 1 to 100%, preferably from 5 to 70% that of the sum of the volumes of the anode and cathode compart ments.

The aqueous solution containing chloride ions is normally a water solution of sodium chloride, although potassium and other soluble chlorides, e.g., magnesium chloride, sometimes also may be utilized, at least in part. However, it is preferable to employ the alkali metal chlorides and of these sodium chloride is the best. Sodium and potassium chlorides include cations which do not form insoluble salts or precipitates and which produce stable hydroxides. The concentration of sodium chloride in a brine charged will usually be as high as feasible, normally being from 200 to 320 grams per liter for sodium chloride and from 200 to 360 g./l. for potassium chloride, with intermediate figures for mixtures of sodium and potassium chlorides. The electrolyte may be neutral or acidified to a pH in the range of about 1 to 6, acidification normally being effected with a suitable acid such as hydrochloric acid. Charging of the brine is to the anolyte compartment, usually at a concentration of 200 to 320 g./l., most preferably of 250 to 300 g./l. Intracompartmental recirculations of the various compartment contents are often desirable to maintain the concentrations uniform throughout.

The presently preferred cation permselective membrane is of a hydrolyzed copolymer of perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The perfluorinated hydrocarbon is preferably tetrafluoroethylene, although other perfluorinated and saturated and unsaturated hydrocarbons of 2 to 5 carbon atoms may also be utilized, of which the monoolefinic hydrocarbons are preferred, especially those of 2 to 4 carbon atoms and most especially those of 2 to 3 carbon atoms, e.g., tetrafluoroethylene, hexafluoropropylene. The sulfonated perfluorovinyl ether which is most useful is that of the formula FSO CF C- F- OCF(CF )CF OCF=CF Such a material, named as perfluoro[2-(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to henceforth as PSEPVE, may be modified to equivalent monomers, as by modifying the internal perfluorosulfonylethoxy component to the corresponding propoxy component and by altering the propyl to ethyl or buty l, plus rearranging positions of substitution of the sulfonyl thereon and utilizing isomers of the perfluorolower alkyl groups, respectively. However, it is most preferred to employ PSEPVE.

The method of manufacture of the hydrolyzed copolymer is described in Example XVI] of US. Pat. No. 3,282,865 and an alternative method is mentioned in Canadian Pat. No. 849,670, which also discloses the use of the finished membrane in fuel cells, characterized therein as electrochemical cells. The disclosures of such patents are hereby incorporated herein by reference. In short, the copolymer may be made by reacting PSEPVE or equivalent with tetrafluoroethyleneor equivalent in desired proportions in water at elevated temperature and pressure for over an hour, after which time the mix is cooled. It separates into a lower perfluoroether layer and an upper layer of aqueous medium with dispersed desired polymer. The molecular weight is indeterminate butithe equivalent weight is about 900 to 1,600 preferably l,l00 to 1,400 and the percentage of PSEPVE or corresponding compound is about 10 to 30%, preferably l5 to 20% and most preferably about 17%. The unhydrolyzed copolymer may be compression molded at high temperature and pressure to produce sheets or membranes, which may vary in thickness from 0.02 to 0.5 mm. These are then further treated to hydrolyze pendant SO F groups to SO;,H groups, as by treating with 10% sulfuric acid or by the methods of the patents previously mentioned. The presence of the SO,,H groups may be verified by titration, as described in the Canadian patent. Additional details of various processing steps are described in Canadian Pat. No. 752,427 and US. Pat. No. 3,041,317, also hereby incorporated by reference.

Because it has been found that some expansion accompanies hydrolysis of the copolymer it is preferred to position the copolymer membrane after hydrolysis onto a frame or other support which will hold it in place in the electrolytic cell. Then it may be clamped or cemented in place and will be true, without sags. The membrane is preferably joined to the backing tetrafluoroethylene or other-suitable filaments prior to hydrolysis, when it is still thermoplastic; and the film of copolymer covers each filament, penetrating into the spaces between them and even around behind them, thinning the films slightly in the process, where they cover the filaments.

The membrane described is far superior in the present processes to all other previously suggested membrane materials. It is more stable at elevated temperatures, e.g., above C. It lasts for much longer time periods in the medium of the electrolyte and the caustic product and does not become brittle when subjected to chlorine at high cell'ter'nperatures. Considering the savings in time and fabrication costs, the present membranes are more economical. The voltage drop through the membranes is acceptable and does not become inordinately high, as it does with many other membrane materials, when the caustic concentration in the cathode compartment increases to above about 200 g./l. of caustic. The selectivity of the membrane and its compatibility with the electrolyte do not decrease detrimentally as the hydroxyl concentration in the catholyte liquor increases, as has been noted with other membrane materials. Furthermore, the caustic efficiency of the electrolysis does not diminish as significantly as it does with other membranes when the hydroxyl ion concentration in the catholyte increases. Thus, these differences in the present process make it practicable, whereas previously described processes have not at tained commercial-acceptance. While the more preferred copolymers are those having equivalent weights of 900 to 1,600, with 1,100 to 1,400 being most preferred, some useful resinous membranes produced by the present method may be of equivalent weights from 500 to 4,000. The medium equivalent weight polymers are preferred because they are of satisfactory strength and stability, enable better selective ion exchange to take place and are of lower internal resistances, all of which are important to the present electrochemical cell.

Improved versions of the above-described copolymers may be made by chemical treatment of surfaces thereof, as by treatments to modify the -SO l-l group thereon. For example, the sulfonic group may be altered on the membrane to produce a concentration gradient or may be replaced in part with a phosphoric or phosphonic moiety. Such changes may be made in the manufacturing process or after production of the membrane. When effected as a subsequent surface treatment of a membrane the depth of treatment will usually be from 0.001 to 0.01 mm. Caustic efficiencies of the invented processes, using such modified versions of the present improved membranes can increase about 3 to 20%, often about to Exemplary of such treatments is that described in French patent publication 2,152,194 of Mar. 26, 1973 in which one side of the membrane is treated with Nl-l to form SO Nl-l groups.

In addition to the copolymers previously discussed, including modifications thereof, it has been found that another type of membrane material is also superior to prior art films for applications in the present processes. Although it appears that tetrafiuoroethylene (TFE) polymers which are sequentially styrenated and sulfonated are not useful for making satisfactory cationactive permselective membranes for use in the present electrolytic processes it has been established that perfluorinated ethylene propylene polymer (FEP) which is styrenated and sulfonated makes a useful membrane. Whereas useful lives of as much as three years or more (that of the preferred copolymers) may not be obtained, the sulfostyrenated FEPs are surprisingly resistant to hardening and otherwise failing in use under the present process conditions.

To manufacture the sulfostyrenated FEP membranes a standard FEP, such as manufacture by E. I. DuPont de Nemours & Co., Inc., is styrenated and the styrenated polymer is then sulfonated. A solution of styrene in methylene chloride or benzene at a suitable concentration in the range of about 10 to is prepared and a sheet of PEP polymer having a thickness of about 0.02 to 0.5 mm., preferably 0.05 to 0.15 mm., is dipped into the solution. After removal it is subjected to radiation treatment, using a cobalt radiation source. The rate of application may be in the range of about 8,000 rads/hr. and a total radiation application is about 0.9 megarads. After rinsing with water the phenyl rings of the styrene portion of the polymer are monosulfonated, preferably in he para position, by treatment with chlorosulfonic acid, fuming sulfuric acid or S0 Preferably, chlorosulfonic acid in chloroform is utilized and the sulfonation is completed in about /2 hour.

Examples of useful membranes made by the de scribed process are products of RAl Research Corporation, Hauppauge, New York, identified as 18ST12S and 16ST13S, the former being 18% styrenated and having two-thirds of the phenyl groups monosulfonated and the latter being 16% styrenated and having thirteen-sixteenths of the phenyl groups monosulfonated. To obtain 18% styrenation a solution of 17%% of styrene in methylene chloride is utilized and to obtain the 16% styrenation a solution of 16% of styrene in methylene chloride is employed.

The products resulting compare favorably with the preferred copolymers previously described, giving voltage drops of about 0.2 volt each in the present cells at a current density of 2 amperes/sq. in., the same as is obtained from the copolymer.

The membrane walls will normally be from 0.02 to 0.5 mm. thick, preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm. When mounted on a polytet rafluoroethylene, asbestos, titanium or other suitable network, for support, the network filaments or fibers will usually have a thickness of 0.01 to 0.5 mm., preferably 0.05 to 0.15 mm., corresponding to up to the thickness of the membrane. Often it will be preferable for the fibers to be less than half the film thickness but filament thicknesses greater than that of the film may also be successfully employed, e.g., 1.1 to 5 times the film thickness. The networks, screens or cloths have an area percentage of openings therein from about 8 to 80%, preferably 10 to and most preferably 30 to 70%. Generally the cross sections of the filaments will be circular but other shapes, such as ellipses, squares and rectangles, are also useful. The supporting network is preferably a screen or cloth and although it may be cemented to the membrane it is preferred that it be fused to it by high temperature, high pressure compression before hydrolysis of the copolymer. Then, the membrane-network composite can be clamped or otherwise fastened in place in a holder or support.

The material of construction of the cell body may be conventional, including concrete or stressed concrete lined with mastics, rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic acid based polyester, polypropylene, polyvinyl chloride, TFE or other suitable plastic or may be similarly lined boxes of other structural materials. Substantially selfsupporting structures, such as rigid polyvinyl chloride, polyvinylidene chloride, polypropylene or phenol formaldehyde resins may be employed, preferably reinforced with moldedin fibers, cloths or webs of glass filaments, steel, nylon,

etc.

The electrodes of the cell can be made of any electrically conductive material which will resist the attack of the various cell contents. In general, the cathodes are made of graphite, iron, lead dioxide on graphite or titanium, steel or noble metal, such as platinum, iridium, ruthenium or rhodium. Of course, when using the noble metals, they may be deposited as surfaces on conductive substrates, e.g., copper, silver, aluminum, steel, iron. The anodes are also of materials or have surfaces of materials such as noble metals, noble metal alloys, noble metal oxides, noble metal oxides mixed with valve metal oxides, e.g., ruthenium oxide plus titanium dioxide, or mixtures thereof, on a substrate which is conductive. Preferably, such surfaces are on or with a valve metal and connect to a conductive metal, such as those described. Especially useful are platinum, platinum on titanium, platinum oxide on titanium, mixtures of ruthenium and platinum and their oxides on titanium and similar surfaces on other valve metals, e.g., tantalum. The conductors for such materials may be aluminum, copper, silver, steel or iron, with copper being much preferred. A preferable dimensionally stable anode is ruthenium oxide-titanium dioxide mixture on a titanium substrate, connected to a copper conductor.

The voltage drops from anodes to cathodes are usually in the range of about 2.3 to volts, although sometimes they are slightly more than 5 volts, e.g., up to 6 volts. Preferably, they are in the range of 3.5 to 4.5

volts. The current densities, while they may be from 0.5

to 4 amperes per square inch of electrode surface, are preferably from 1 to 3 amperes/sq. in. and ideally about 2 amperes/sq. in. The voltage ranges are for perfectly aligned electrodes and it is understood that where such alignment is not exact, as in laboratory units, the voltages can be up to about 0.5 volt higher.

Among the important advantages of the present invention is in the production of concentrated caustic, low in chloride concentration, without the need for dilute caustic being an end product of the method. Yet, this is done at a comparatively high efficiency due to the utilization of the three-compartment cell and the initial manufacture of dilute caustic in the buffer compartment thereof. The use of the buffer compartment and the presence of dilute caustic in it diminishes the pressure on caustic ions to penetrate into the anode compartment of that cell and thereby improves efficiency because less oxygen or other relatively useless product is manufactured in the anolyte than would be the case were more hydroxide to penetrate into it. The pressre of the caustic can also be diminished by feeding additional water to the compartment and making a weaker caustic therein, e.g., one of as little as 25 to 50 g./l. concentration. However, the improved efficiency of this operation must be balanced against the employment of so weak a caustic as a feed to the catholyte of the two-compartment cell.

It is desirable that the anolyte of both cells should be acidic so that it can react with any hydroxyl entering the anode compartment from the buffer zone, preventing oxygen formation. A pH in the ranges of 1 to 6 can be used, the range of l to 5 is preferred and 2 to 4 is best. Of course, the buffer solution and catholyte pHs are 14. The temperature of the electrolyte will be maintained at less than 105C., preferably being 20 to 95C., more preferably being 50 to 95C. and most preferably being about 65 to 95C. Electrolyte temperatures may be controlled by recirculation of portions thereof and by regulations of proportions of feeds to the various zones and the temperatures of such feeds. When temperatures cannot be lowered sufficiently by recirculation or feed control, refrigeration or other cooling means or liquids may also be employed. For example. feeds of diluting water to the buffer companment and dilute caustic to the catholyte compartment of the two-compartment cell and any recirculating anolyte employed may be cooled to about 10to 20C., preferably to about 10C., before admission to the compartment and recirculating electrolyte moving essentially intracompartmentally may also be cooled. alternatively, cooling may be merely by exposure of the liquids to ambient conditions before they enter or reenter the cells.

The greatly improved current efficiencies of the present process are attributable in large part to the 90 to 97% chlorine current efficiency and over often over caustic current efficiency obtainable in the three-compartment cell due to the use of the buffer compartment therein. It has been found that caustic efficiency (Faradaic) 'decreases as caustic concentration of the buffer effluent increases, apparently being essentially a straight line function of concentration from at 73 g./l. to 82% at g./l., then dropping off more sharply to 72% at g./l.

The high concentration caustic solution made is free of chloride or essentially free thereof, often containing as little as 0.1 to .10 g./l. and usually about one g./l. thereof, with the caustic concentration in the 250 to 400 or 250 to 450 g.'/l. range. Such caustic concentrations may be further increased by evaporation and comparatively little thermal energy is needed to raise them to 50%. A possible disadvantage of the present method, the production of chlorate in the anolyte of the two-compartment" cell, is even convertible to an advantage, when that anolyte effluent is fed to chlorate cells. Alternatively, the chlorate may be separated from the recirculating anolyte and may be commercially utilized. I

The present cells may be incorporated in large or small plants, thus producing usable caustic while making from 20 to 1,000 tons per day of chlorine or equivalent and in all cases efficiencies obtained can be such as to make the process economically desirable. It is highly preferred however, that the installation should be located near to and be used in conjunction with a pulp bleaching plant so that the chlorine and chlorate, if any of the latter salt is produced, may be used together with the concentrated caustic in wood pulp bleaching or in the production of bleaching agents, e.g., chlorine dioxide. Of course, the caustic and chlorine manufactured may also be marketed.

The following examples illustrate but do not limit the innvention. Unless otherwise indicated, all parts are by weight and all temperatures are in C.

, EXAMPLE 1 The three-compartment and two-compartment cells of the Figure are employed, with the changes described herein, to produce chlorine, hydrogen and concentrated sodium hydroxide solution from an aqueous sodium chloride solution. As illustrated, for simplicity, a single three-compartment and a single twocompartment cell are employed but in actual practice the ratio of threecompartment cells to twocompartment cells will be about 1.5, with the feeds to the various cells and discharges from them often being through common lines.

The cell walls are-of asbestos filled polypropylene or polypropylene or may be of steel lined with unplasticized polyvinyl chloride and in some instances, polypropylene. Polyesters, such as chlorendic acid based polyesters, e.g., Hetron, made by Hooker Chemical Corp., may be used as a wall lining, too. Rubber or other synthetic plastic gaskets may act as seals between cell walls, covers and other parts. The electrodes are in contact with the membranes separating the buffer compartment from the electrode compartments of the three-compartment cell and such membranes are cation-active permselective membranes manufactured by E. I. DuPont de Nemours & Company, lnc. and sold as their XR-type membranes. The membranes are 7 mils thick, (about 0.2 mm.) and are joined to a backing or supporting network of polytetrafluoroethylene (Teflon) filaments of a diameter of about 0.1 mm., woven into a cloth which has an area percentage of openings therein of about 22%. They were initially flat and were fused onto the screen or cloth of Teflon by high temperature, high compression pressing, with some of the membrane portions actually flowing around the filaments during the fusion process to lock onto the cloth, without thickening the membrane between the cloth filaments.

, The material of the XR-type permselective membrane is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether. The copolymer is of tetrafluoroethylene and FSO CF CF OCF(CF )CF OCF=CF and has an equivalent weight in the 900 to 1,600 range, about 1,250. The electrodes are in contact with the buffer membranes, with the flatter sides of the membranes facing the contacting electrodes. In some experiments spacings of 0.01 to 5 mm. between the electrodes and the membranes are utilized and satisfactory results are obtained but the present arrangement, and the absence of spacings is preferred. The same membranes are employed in both the three-compartment and twocompartment cells.

In the three-compartment cells the buffer compartment is about 6 mm. wide and the electrodes are positioned against the membranes, making the interelectrode distance essentially the same. The volume ratios of the anode compartment buffer compartment cathode compartment are about 10:1:10 and the anode and cathode compartment of the two-compartment cell are of about the same volume, with the permselective membrane being centrally located and electrode gaps (from anode to cathode) being 1 to 10 mm.

The anodes utilized are of a mixture of ruthenium and titanium oxides on titaniumcommunicated with current sources by titanium-clad copper members. The titanium base for the anode is titanium mesh, about 1 mm., in diameter and with about 50% open area, coated with a 1:3 ruthenium oxide, titanium oxide mixture-about 1 mm. thick. The cathodes are of a mild steel wire mesh, essentially 1 mm. in equivalent diameter, having about 35% open area, and are communicated with negative electrical sources or sinks by a copper conductor.

The anode compartments of the cells are filled with a nearly saturated salt solution or brine of sodium chloride at about a concentration and the cathode and buffer compartment are filled with water, initially containing'a small quantity of salt or brine to improve conductivity. The current is turned on and the chlorine and hydrogen produced in the cells are taken off. Water is fed to the buffer compartment of the threecompartment cell to maintain the concentration of sodium hydroxide therein low and at the desired concentrations dilute and more concentrated sodium hydroxide solutions are removed from the buffer compartment and the cathode compartment, respectively, of the three-compartment cell. That taken off the buffer compartment is fed to the cathode compartment of the the sodium chloride content of the withdrawn anolyte is about 22% by weight and that of the returned anolyte from the resaturator is about 25%. A proportion of the recirculating electrolyte may bypass the resaturator. As

illustrated in the drawing, a similar recirculation and resaturation in the anolyte of the two-compartment cell is effected, utilizing the same resaturator and by valve adjustment the proportions of flows to each of the anolyte compartments of the three-compartment and twocompartment cells are regulated. Of course, if desired, separately circulating systems may be employed. As illustrated, provision is made for taking off a proportion of the anolyte, especially from the two-compartment cell, when desired. The removal of anolyte may be effected to enable the crystallization from it of any excess chlorate content.

In a plant rated at tons of chlorine per day, including 2O l50-kiloampere cells, 12 of which are threecompartment cells and 8 of which are twocompartment cells, 25% sodium chloride solution is fed to the three-compartment cell anode compartment, 22% sodium chloride content anolyte is removed from it and is recirculated back to the anode compartment after rejuvenation thereof by dissolving of solid salt in such solution in the resaturator. Acidification is also effected at this time, to a pH of 3. A current density of 2 amperes/sq. in. is applied to the electrodes at a potential drop of 4.2 volts. Chlorine is produced at a current efficiency of about 96% and it contains essentially no free oxygen. The caustic efficiency of the threecompartment cell is about 90% and it produces 36 tons of sodium hydroxide per day at a concentration of 400 g./l. and 18 tons per day at a concentration of g./l. The more concentrated caustic contains only 0.5 g./l. of NaCl. It is sent to an evaporator and is raised to 50% concentration, although in some instances it is sold directly as 400 g./l. solution or is utilized in such form for woodpulp treatment.

In the two-compartment cell the gap between the anode and cathode is approximately 2 mm., the chloride solution feeds are the same as for the threecompartment cell and in addition to the caustic charge to the cathode compartment and thefeed solution to the anode compartment, sometimes a water feed is sent to the cathode compartment to regulate the concentration of the product thereof. Operating at 3.6 volts and 2 amperes/sq. in. the eight cells produce a total of 42 tons of sodium hydroxide per day of a concentration of 400 g./l. and a sodium chloride concentration of 0.8 g./l. It is noted that the total hydroxide production for the 20 cells is 78 tons of sodium hydroxide per day, which corresponds to a total plant caustic efficiency of about 78%. The three-compartment cells operate at a caustic efficiency of about 85-90% and the twocompartment cells operate at about 60% caustic efficiency. When the anolyte is not acidified the chlorine produced in the two-compartment cells may be kept separate from that of the three-compartment cells because of its oxygen or impurity content being more than would have been the case if it was acidified. However, it contains less than 6% of oxyen. When hydrochloric acid is added to the anolyte of the twocompartment cells in sufficient quantity to neutralize the migrating caustic the chlorine from the cell can be as pure as that from a threecompartment cell.

The products obtained are used in pulping and in pulp bleaching operations, especially in the treatment of groundwood pulps. In some instances the caustic and chlorine are reacted to form bleaching agents, such as hypochlorite and chlorate, the latter of which may be subsequently treated to produce chlorine dioxide for pulp bleaching.

In a modification of the described process, after a buildup of chlorate in the anolyte of the twocompartment cell (which is not mixed in with anolytes from three-compartment cells) such anolyte is fed to a chlorate cell or cells where the chloride therein is converted to chlorate and the chlorate content is usefully additive.

In other modifications of the process the thicknesses of the membranes are increased to to 14 mils, at which caustic efficiencies increase but voltage drops are also greater. Thus, although the membranes of greater thicknesses are operative it is preferred to employ the 7 mil membranes in these reactions. When the membranes thickness is decreased to 4 mils the process is satisfactory but caustic efficiency is diminished somewhat.

EXAMPLE 2 The process of Example 1 is repeated, employing 10 mil membranes of membrane materials identified as l8STl2S and 16ST13S, respectively, made by RAI Research Corporation, in replacement of the hydrolyzed copolymer of tetrafluoroethylene and sulfonated perfluorovinyl ether. The former of the RAI products is a sulfostyrenated FEP in which the FEP is 18% styrenated and has two-thirds of the phenyl groups thereof monosulfonated, and the latter is 16% styrenated and has thirteen-sixteenths of the phenyl groups monosulfonated. Under operating conditions the membranes stand up better than other available cation-active permselective membranes but not as well as the membranes of Example 1. After such use their characteristics, e.g., physical appearances, uniformity, voltage drop, are better than other cation-active permselective membrane materials available (except the hydrolyzed copolymers of perfluorinated hydrocarbons and fluorosulfonated perfluorovinyl ethers) but especially in the anode compartment where chlorine attacks the membrane lengthening of operating life is desirable. However, in the cathode compartment the RAI membranes are decidedly better in operation than comparable commercial products, except for those of the MX type.

In a variation of this procedure the operating temperature is changed to 80C. and although efficiencies are somewhat lower the reactions are satisfactorily operative. Similar good results are also obtained when the surface of the cathode is changed to platinum or graphite and the surface of the anode is changed to platinum or ruthenium oxide (on titanium). The concentrated caustic and chlorine of this example are used to make sodium chlorate and the 400 g./l. caustic, without further concentration, is employed for the pulping of groundwood.

EXAMPLE 3 The experiment of Example I is repeated, using a single three-compartment cell and a single twocompanment cell and feeding two-thirds of the dilute caustic production of the three-compartment to the cathode compartment of the two-compartment cell. Otherwise, the conditions obtaining are the same and the results thereof are the same although production of concentrated caustic falls off accordingly and some dilute caustic remains to be disposed of, (it is used in the pulping of groundwood). When further changes are made in the operating conditions, as by varying the operating temperatures, chloride feed concentrations, ratios of compartment volumes, the screen or cloth backing for the membranes, the current densities, voltages and/or anolyte pI-Is, within the ranges hereinbefore described, satisfactory products are made at overall caustic current efficiencies greater than The efficiencies in such methods are further increased by utilizing a modified membrane of the hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether type, as previously described. In other experiments plant sizes are increased or diminished by utilization of more or fewer cells and/or by the changings of cell sizes. In such experiments the high concentration caustic solution produced contains from 375 to 425 g./l. of sodium hydroxide and the feed to the two-compartment from the threecompartment cell is from to g./l. sodium hydroxide. In still other variations of the examples the continuous processes described are changed to batch operations and while efficiencies drop, the processes are operative and produce the same high concentration caustics of good purities.

The invention has been described with respect to working examples and illustrative embodiments but is not to be limited to "these because it is evident that one of ordinary skill in the art will be able to utilize substitutes and equivalents without departing from the spirit of the invention or the scope of the claims.

What is claimed is:

l. A method for electrolytically manufacturing a concentrated hydroxide solution containing over 250 but less than 450 g./l. of sodium hydroxide or equivalent hydroxide which comprises making concentrated and dilute aqueous hydroxide solutions simultaneously by electrolyzing an aqueous solution containing halide ions in an electrolytic cell having at least three compartments therein, an anode, a cathode, at least two cation-active permselective membranes of a polymeric material selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether and a sulfostyrenated perfluorinated ethylene propylene polymer, defining anode and cathode side walls of a buffer compartment or compartments between anode and cathode compartments, and such walls, with walls thereabout, defining anode and cathode compartments, while adding water to the buffer compartment at such a rate as to produce a dilute hydroxide solution therein at the same time that a more concentrated hydroxide solution, containing over 250 but less than 450 g./l. of sodium hydroxide or equivalent is produced in the cathode compartment, while maintaining a high caustic efficiency, removing the dilute hydroxide from the buffer compartment and feeding it to the cathode compartment of a two-compartment electrolytic cell, having anode and cathode compartments separated by a cation-active permselective membrane of a polymeric material selected from the group consisting of a hydrolyzed copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether, and a sulfostyrenated perfluorinated ethylene propylene polymer, in which cell, in the anode compartment thereof, an aqueous solution containing halide ions is electrolyzed, and withdrawing from the catholyte compartment of the two-compartment cell concentrated hydroxide solution containing more than 250 but less than 450 g./l. of sodium hydroxide or equivalent, so that the cathode compartment efficiency of the combined processes is above 70%.

2. A method according to claim 1 wherein the electrolytic cells are three-compartment and twocompartment cells, the walls between the buffer compartment of the three-compartment cell and the anode and cathode compartments thereof and the wall between the anode and cathode compartments of the twocompartrnent cell are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FSO CF CF OCF(CF )C- F OCF=CF which copolymer has an equivalent weight of about 900 to 1,600, and the addition of water to the buffer compartment of the three-compartment cell is at such a rate that the dilute hydroxide removed from the buffer compartment and fed to the cathode compartment of the two-compartment electrolytic cell is at a concentration greater than 50 g./l.

3. A method according to claim 2 wherein sodium hydroxide, chlorine and hydrogen are made from an aqueous solution of sodium chloride, the concentrated sodium hydroxide solution removed from the cathode compartment of the three-compartment and twocompartment cells is at a concentration over 350 g./l. and the concentration of the sodium hydroxide solution withdrawn from the buffer compartment of the threecompartment cell and fed to the cathode compartment of the two-compartment cell is from 50 to 200 g./l., the permselective membranes are about 0.02 to 0.5 mm. thick, the concentration of sodium chloride in the anode compartments of the three-compartment and two-compartment cells is from about 200 to 320 g./l., the pH of the anolytes in both cells is about 1 to 5 and the temperatures of the anolytes, catholytes and buffer compartment solution are less than 105C.

4. A method according to claim 3 wherein the permselective membranes are mounted on networks of a material selected from the group consisting of polytetrafluoroethylene, asbestos, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which networks have area percentages of openings therein from about 8 to 80%, the temperatures of the anolytes, catholytes and buffer compartment solution are in the range of to 95C, the surfaces of the cathodes are of a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel, the surfaces of the anodes are of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, or mixture thereof, on a valve metal, the voltage is from about 2.3 to 6 volts and the current density is from about 0.5 to 4 amperes per square inch of electrode surface.

5. A method according to claim 4 wherein the networks are screens of cloths of polytetrafluoroethylene filaments having a thickness of 0.1 to 0.3 mm., the membrane walls are from 0.1 to 0.3 mm. thick, the polytetrafluoroethylene filament thickness is less than or equal to that of the membrane walls, thecopolymer equivalent weight is from about 1,100 to 1,400, the cathodes are of steel, the anodes are of ruthenium oxide on titanium, the aqueous sodium chloride solution electrolytes are at a concentration of about 250 to 320 g./l., the pH of the anolyte is from to 4 and the temperatures of the anolytes, catholytes and buffer compartment solution are from 65 to 95C.

6. A method according to claim 5 wherein the aque ous solution of sodium hydroxide withdrawn from the buffer compartment of the three-compartment cell is at a concentration of about 100 to 150 g./l., the concentrations of the catholytes withdrawn from the threecompartment and two-compartment cells are about 375 to 425 g./l. and the caustic efficiency of the combined process is over 7. A method according to claim 6 wherein a plurality of three-compartment and two-compartment cells is employed, the three-compartment cells operating at about caustic efficiency or more and producing buffer compartment solution containing about 125 g./l. sodium hydroxide and the two'compartment cells oper ating at about 60% caustic efficiency or more, with the solution removed from the cathode compartment of both cells being at about 400 g./l. sodium hydroxide, the proportion of three-compartment cells to twocompartment cells is about 1.5 and the buffer compartment solution withdrawn from three threecompartment cells is fed to the catholytes of two twocompartment cells.

8. A method according to claim 3 wherein the aqueous solution of sodium hydroxide withdarwn from the buffer compartment of the three-compartment cell is at a concentration of about to 150 g./l., the concentrations of the catholytes withdrawn from the threecompartment and two-compartment cells are about 375 to 425 g./l. and the caustic efficiency of the combined process is over 75%.

9. A method according to claim 3 wherein a plurality of three-compartment and two-compartment cells is employed, the three-compartment cells operating at about 90% caustic efficiency or more and producing buffer compartment solution containing about g./l. sodium hydroxide and the two-compartment cells operating at about 60% caustic efficiency or more, with the solution removed from the cathode compartments of both cells being at about 400 g./l. sodium hydroxide, the proportion of three-compartment cells to twocompartment cells is about 1.5 and the buffer compartment solution withdrawn from three threecompartment cells is fed to the catholytes of two two compartment cells. 

1. A METHOD FOR ELECTROLYTICALLY MANUFACTURING A CONCENTRATED HYDROXIDE SOLUTION CONTAINING OVER 250 BUT LESS THAN 450 G./L. OF SODIUM HYDROXIDE OR EQUIVALENT HYDROXIDE WHICH COMPRISES MAKING CONCENTRATED AND DILUTE AQUEOUS HYDROXODE SOLUTIONS SIMULTANEOUSLY BY ELECTROLYZING AN AQUEOUS SOLUTION CONTAINING HALIDE IONS IN AN ELECTROLYTIC CELL HAVING AT LEAST THREE COMPARTMENTS THEREIN, AN ANODE, A CATHODE, T LEAST TWO CATION-ACTIVE PERMSELECTIVE MEMBRANES OF A POLYMERIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF A HYDROLYZED COPOLYMER OF A PERFLUORINATED HYDROARBON AND A FLUOROSULFONATED PERFLUOROVINYL ETHER AND A SULFOSTYRENATED PERFLUORINATED ETHYLENE PROPYLENE POLYMER, DEFINING ANODE AND CATHODE SIDE WALLS OF A BUFFER COMPARTMENT OF COMPARTMENTS BETWEEN ANODE AND CATHODE COMPARTMENTS, AND SUCH WALLS, WITH WALLS THEREABOUT, DEFINING ANODE AND CATHODE COMPARTMENTS, WHILE ADDING WATER TO THE BUFFER COMPARTMENT AT SUCH A RATE AS TO PRODUCE A DILUTE HYDROXIDE SOLUTION THEREIN AT THE SAME TIME THAT A MORE CONCENTRATED HYDROXIDE SOLUTION, CONTAINING OVER 250 BUT LESS THAN 450 G./L. OF SODIUM HYDROXIDE OR EQUIVALENT IS PRODUCED IN THE CATHODE COMPARTMENT, WHILE MAINTAINING A HIGH CAUSTIC EFFICIENCY, REMOVING THE DILUTE HYDROXIDE FROM THE BUFFER COMPARTMENT AND FEEDING IT TO THE CATHODE COMPARTMENT OF A TWO-COMPARTMENT ELECTROLYTIC CELL, HAVING ANODE AND CATHODE COMPARTMENTS SEPARATED BY A CATION-ACTIVE PERMSELECTIVE MEMBRANE OF A POLYMERIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF A HYDROLYZED COPOLYMER OF A PERFLUORINATED HYDROCARBON AND A FLUOROSULFONATED PERFLUOROVINYL ETHER, AND A SULFOSTYRENATED PERFLUORINATED ETHYLENE PROPYLENE POLYMER, IN WHICH CELL, IN THE ANODE COMPARTMENT THEREOF, AN AQUEOUS SOLUTION CONTAINING HALIDE IONS IS ELECTROLYZED, AND WITHDRAWING FROM THE CATHOLYTE COMPARTMENT OF THE TWO-COMPARTMENT CELL CONCENTRATED HYDROXIDE SOLUTION CONTAINING MORE THAN 250 BUT LESS THAN 450 G./L. OF SODIUM HYDROXIDE OR EQUIVALENT, SO THAT THE CATHODE COMPARTMENT EFFICIENCY OF THE COMBINED PROCESSES IS ABOVE 70%.
 2. A method according to claim 1 wherein the electrolytic cells are three-compartment and two-compartment cells, the walls between the buffer compartment of the three-compartment cell and the anode and cathode compartments thereof and the wall between the anode and cathode compartments of the twocompartment cell are of a hydrolyzed copolymer of tetrafluoroethylene and a fluorosulfonated perfluorovinyl ether of the formula FSO2CF2CF2OCF(CF3)CF2OCF CF2, which copolymer has an equivalent weight of about 900 to 1,600, and the addition of water to the buffer compartment of the three-compartment cell is at such a rate that the dilute hydroxide removed from the buffer compartment and fed to the cathode compartment of the two-compartment electrolytic cell is at a concentration greater than 50 g./l.
 3. A method according to claim 2 wherein sodium hydroxide, chlorine and hydrogen are made from an aqueous solution of sodium chloride, the concentrated sodium hydroxide solution removed from the cathode compartment of the three-compartment and two-compartment cells is at a concentration over 350 g./l. and the concentration of the sodium hydroxide solution withdrawn from the buffer compartment of the three-compartment cell and fed to the cathode compartment of the two-compartment cell is from 50 to 200 g./l., the permselective membranes are about 0.02 to 0.5 mm. thick, the concentration of sodium chloride in the anode compartments of the three-compartment and two-compartment cells is from about 200 to 320 g./l., the pH of the anolytes in both cells is about 1 to 5 and the temperatures of the anolytes, catholytes and buffer compartment solution are less than 105*C.
 4. A method according to claim 3 wherein the permselective membranes are mounted on networks of a material selected from the group consisting of polytetrafluoroethylene, asbestos, perfluorinated ethylene propylene polymer, polypropylene, titanium, tantalum, niobium and noble metals, which networks have area percentages of openings therein from about 8 to 80%, the temperatures of the anolytes, catholytes and buffer compartment solution are in the range of 20* to 95*C, the surfaces of the cathodes are of a material selected from the group consisting of platinum, iridium, ruthenium, rhodium, graphite, iron and steel, the surfaces of the anodes are of a material selected from the group consisting of noble metals, noble metal alloys, noble metal oxides, mixtures of noble metal oxides with valve metal oxides, or mixture thereof, on a valve metal, the voltage is from about 2.3 to 6 volts and the current density is from about 0.5 to 4 amperes per square inch of electrode surface.
 5. A method according to claim 4 wherein the networks are screens of cloths of polytetrafluoroethylene filaments having a thickness of 0.1 to 0.3 mm., the membrane walls are from 0.1 to 0.3 mm. thick, the polytetrafluoroethylene filament thickness is less than or equal to that of the membrane walls, the copolymer equivalent weight is from about 1,100 to 1,400, the cathodes are of steel, the anodes are of ruthenium oxide on titanium, the aqueous sodium chloride solution electrolytes are at a concentration of about 250 to 320 g./l., the pH of the anolyte is from 2 to 4 and the temperatures of the anolytes, catholytes and buffer compartment solution are from 65* to 95*C.
 6. A method according to claim 5 wherein the aqueous solution of sodium hydroxide withdrawn from the buffer compartment of the three-compArtment cell is at a concentration of about 100 to 150 g./l., the concentrations of the catholytes withdrawn from the three-compartment and two-compartment cells are about 375 to 425 g./l. and the caustic efficiency of the combined process is over 75%.
 7. A method according to claim 6 wherein a plurality of three-compartment and two-compartment cells is employed, the three-compartment cells operating at about 90% caustic efficiency or more and producing buffer compartment solution containing about 125 g./l. sodium hydroxide and the two-compartment cells operating at about 60% caustic efficiency or more, with the solution removed from the cathode compartment of both cells being at about 400 g./l. sodium hydroxide, the proportion of three-compartment cells to two-compartment cells is about 1.5 and the buffer compartment solution withdrawn from three three-compartment cells is fed to the catholytes of two two-compartment cells.
 8. A method according to claim 3 wherein the aqueous solution of sodium hydroxide withdarwn from the buffer compartment of the three-compartment cell is at a concentration of about 100 to 150 g./l., the concentrations of the catholytes withdrawn from the three-compartment and two-compartment cells are about 375 to 425 g./l. and the caustic efficiency of the combined process is over 75%.
 9. A method according to claim 3 wherein a plurality of three-compartment and two-compartment cells is employed, the three-compartment cells operating at about 90% caustic efficiency or more and producing buffer compartment solution containing about 125 g./l. sodium hydroxide and the two-compartment cells operating at about 60% caustic efficiency or more, with the solution removed from the cathode compartments of both cells being at about 400 g./l. sodium hydroxide, the proportion of three-compartment cells to two-compartment cells is about 1.5 and the buffer compartment solution withdrawn from three three-compartment cells is fed to the catholytes of two two-compartment cells. 